The invention involves a magnetic structure which abruptly changes at its critical temperature (Curie temperature) from a magnetic to a non-magnetic state and which can be interrogated and its state determined by appropriate external circuitry. More particularly, the invention pertains to the method of achieving the desired abrupt change at a preselected critical temperature and the utilization of this phenomenon in a non-saturated linear signal propagating apparatus wherein the signal propagating properties of the material are altered sharply after a critical temperature change of less than 0.1.degree. C.
Magnetic materials such as spinel ferrites are well known in the art and have a physical characteristic of a critical temperature known as the Curie temperature at which they lose their magnetic properties. Also, it is generally well known that some properties such as magnetization change gradually with temperature changes whereas other characteristics such as the initial susceptibility may disappear abruptly at the critical temperature. With magnetic devices wherein the initial susceptibility (or permeability) of the magnetic material is important, their performance is generally required to be independent of temperature and therefore they operate at a temperature well below the critical temperature. Such devices include inductors, transformers and the like.
It is not uncommon to measure or to utilize the change in magnetization which changes with temperature to determine the induced voltage of a generator or temperature detector or the like and to control the output. However, such mechanisms have difficulty in that the magnetization has a slow rate of change with temperature at any given point thus creating a large area of uncertainty as to the accuracy of the temperature reading. Thus, such devices inherently have a low sensitivity to temperature changes.
It is also generally known that the critical temperature of a magnetic material is determined by its chemical and crystallographic composition. However, for many magnetic materials, this temperature varies over a relatively broad range. The difficulties associated with the preparation of a material of precisely known composition has heretofore limited control of the critical temperature to a range of several degrees. Thus, before the instant invention it has not been possible to produce a magnetic substance having a predetermined critical temperature except within a relatively broad range of say plus or minus 2.degree. C.